This can be solved by unitary method.
m students-------------------> m pages -----------------> in m days
m students--------------------> m/m pages/day=1 page/day
1 student ---------> 1/m page/day
100 students= 100/m pages/day
GIVEN : 100 students will require 100 pages.
1 day -----------> 100/m pages by 100 students
x days--------------> 100 pages by 100 students.
solve for x by cross multiplication. x=m days .
hence, option D is correct